When working with libcurl in Ubuntu 11.10, you're essentially choosing between two SSL/TLS implementations:

• libcurl4-gnutls-dev: Uses GNUTLS as its cryptographic backend
• libcurl4-nss-dev: Uses Mozilla's NSS (Network Security Services) library

The choice primarily affects:

// Example curl initialization with different backends
CURL *curl = curl_easy_init();
if(curl) {
    // This underlying TLS behavior changes based on backend
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 1L);
    // Additional configuration...
}

GNUTLS (libcurl4-gnutls-dev):

• Generally more permissive with certificate validation
• May work better with self-signed certificates
• Sometimes has better performance with certain protocols

NSS (libcurl4-nss-dev):

• Stricter security policies by default
• Better integration with system certificate stores
• Preferred for environments requiring FIPS compliance

The version difference (3 vs 4) primarily relates to API compatibility. Key points:

// libcurl3 code may need adjustment for libcurl4
// Example of deprecated option in older versions
curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_ANYSAFE);

Major version changes typically introduce breaking API modifications. For new development, always target libcurl4.

For most Ubuntu 11.10 server applications:

sudo apt-get install libcurl4-gnutls-dev

Is generally the safer choice unless you have specific NSS requirements.

If you encounter SSL-related problems after installation:

// Verify which backend is actually being used
curl_version_info_data *ver = curl_version_info(CURLVERSION_NOW);
printf("SSL backend: %s\n", ver->ssl_version);

Remember that some applications may have hard dependencies on a specific backend.

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When working with libcurl in Ubuntu 11.10, you'll encounter two primary development packages:

libcurl4-gnutls-dev - Uses GNUTLS as the SSL backend
libcurl4-nss-dev - Uses Mozilla's NSS as the SSL backend

The core difference lies in their cryptographic implementations:

• Protocol Support: GNUTLS generally supports more protocols (including DTLS) while NSS focuses on mainstream protocols
• Certificate Handling: NSS uses its own certificate store while GNUTLS uses the system's
• Performance: GNUTLS may perform better with certain cipher suites

The version jump from 3 to 4 introduced several breaking changes:

// libcurl3 (deprecated)
#include 
CURL *curl = curl_easy_init();

// libcurl4 (current)
// Same interface but with updated SSL backend options

Key differences include updated API constants, improved SSL verification defaults, and support for newer protocols like HTTP/2.

Here's how to check which backend your installation is using:

#include 
#include 

int main() {
    curl_version_info_data *data = curl_version_info(CURLVERSION_NOW);
    printf("SSL backend: %s\n", data->ssl_version);
    return 0;
}

Compile and run with:

gcc curl_backend_check.c -lcurl -o backend_check
./backend_check

When moving between versions or backends:

1. Test all SSL/TLS connections thoroughly
2. Verify certificate handling behavior
3. Check for any protocol-specific features your application might rely on

To install a specific backend:

# For GNUTLS backend
sudo apt-get install libcurl4-gnutls-dev

# For NSS backend
sudo apt-get install libcurl4-nss-dev

Remember that most applications will work with either backend, but some may have specific requirements.